1. Field of the Disclosure
The present disclosure relates to an optical network capable of changing the frequency of an optical signal and an optical signal modulation method thereof.
2. Description of Related Art
FIG. 1 is a schematic diagram of a conventional optical network. The optical network includes a head-end 110, an optical fiber 120, and a wavelength division multiplexer (WDM) 130. The head-end 110 includes a plurality of laser sources. A single laser source 111 is demonstratively illustrated in FIG. 1. The laser source 111 provides a continuous carrier wave 151. The WDM 112 combines and outputs the continuous carrier waves emitted by all the laser sources through multiplexing. The continuous carrier wave 151 enters the optical fiber 120 via an optical circulator (OC) 113. Ideally, the cross section of an optical fiber should be a perfect round. However, actually, it may not be so perfect. Thus, the continuous carrier wave 151 produces a reflected wave 152 after traveling every small distance in the optical fiber 120, and this reflected wave 152 is the so-called Rayleigh backscattering noise.
At the other end of the optical fiber 120, the WDM 130 separates the continuous carrier waves provided by the laser sources and provides the continuous carrier waves to a plurality of user devices. For example, a user device 140 receives the continuous carrier wave 151 emitted by the laser source 111, and the user device 140 modulates and loads data to the continuous carrier wave 151 to generate an upstream signal 153. The WDM 130 combines and inputs the upstream signals generated by all the user devices into the optical fiber 120 through multiplexing. These upstream signals also produce reflected waves when they pass through the optical fiber 120. These reflected waves are received and re-modulated by the user devices and output toward the head-end 110 as the Rayleigh backscattering noises of the upstream signals. For example, the upstream signal 153 produces a Rayleigh backscattering noise 154.
Foregoing upstream signals are input into a WDM 114 via the OC 113 of the head-end 110. The WDM 114 filters the upstream signals to separate them. The head-end 110 includes a plurality of receivers for receiving all the upstream signals. A single receiver 115 is demonstratively illustrated in FIG. 1, and which receives the upstream signal 153.
Besides the upstream signal 153, the Rayleigh backscattering noise 152 of the continuous carrier wave 151 and the Rayleigh backscattering noise 154 of the upstream signal 153 are also received by the receiver 115, and these signals have the same wavelength. Accordingly, the reception of the upstream signal 153 is interfered by the Rayleigh backscattering noises 152 and 154.